Ecological effects of Tasmanian devil Sarcophilus harrisii declines
Date: 19 July 2021
University of Exeter
PhD in Biological Sciences
Species declines, which are both widespread and worsening, affect the ecological dynamics not only of declining populations, but also the species with which they interact. In the case of top carnivores, their marked declines are triggering trophic cascades whereby their structuring influence on communities is lost, resulting in release ...
Species declines, which are both widespread and worsening, affect the ecological dynamics not only of declining populations, but also the species with which they interact. In the case of top carnivores, their marked declines are triggering trophic cascades whereby their structuring influence on communities is lost, resulting in release of prey species from predation pressure and sympatric carnivores from competitive pressure. Changes in competitive pressure both within and between sympatric species are predicted to result in changes in the ecological niches of individuals and populations, according to the niche variation hypothesis. Investigating the niche dynamics of communities experiencing top carnivore loss allows us to test theoretical predictions of how ecological niches respond to competition, as well as furthering our understanding of the role and function of top carnivores. In this thesis, I have explored the effects of Tasmanian devil Sarcophilus harrisii decline, following the emergence of a transmissible cancer, devil facial tumour disease (DFTD). I used stable isotope analysis to characterise the trophic ecology of Tasmanian devils, a top marsupial carnivore, and to investigate the impact of disease and population decline on the trophic niches of devil individuals and populations, and of spotted-tailed quolls Dasyurus maculatus, a closely-related sympatric marsupial carnivore. I first quantified patterns of isotopic variation within a Tasmanian devil population. Using δ13C and δ15N values from whisker tissue samples collected from Tasmanian devils at Wilmot, Tasmania, I demonstrated that both δ13C and δ15N, and group isotopic niche breadth decreased with increasing age in weaned Tasmanian devils. By characterising the isotopic niche breadth of a subset of individuals, I showed that individual niche breadth also decreased with increasing age, and revealed an isotopic signature of weaning in young Tasmanian devils. Next, I explored the impact of DFTD on the trophic ecology of infected Tasmanian devils. I tested whether DFTD progression, measured as tumour volume, affected δ13C and δ15N values of whiskers of Tasmanian devils collected at six sites across Tasmania. I found isotope values did not change with increasing tumour volume, except at one site, Freycinet, which showed differences in the relative abundance of three common prey species compared to our other sites, based on species distribution models. I also showed that whisker isotope values of individual Tasmanian devils sampled before and after detection of clinical signs of disease do not differ, when compared to healthy control individuals. I conclude that, according to stable isotope analysis, devils do not generally change their diet in response to DFTD but that contextual ecological factors such as prey availability may elicit or allow a change in diet as the disease progresses. I then used Bayesian stable isotope mixing models to estimate the proportional contribution of prey groups to the diet of Tasmanian devils. I examined variation in δ13C and δ15N values from Tasmanian devils and their putative prey species at six sites, and concluded only one site was suitable for mixing model analysis. The results suggested that the devil population at Woodbridge, Tasmania, consumed similar amounts of Tasmanian pademelon Thylogale billardierii and small mammals, and fewer Bennett’s wallabies Macropus rufogriseus. I highlight the importance of further research to quantify trophic discrimination factors in marsupial species, and the difficulty in sampling the prey base of opportunistic carnivores, with large ranges relative to their prey. Considering the potential impact of Tasmanian devil decline on community niche dynamics, I examined the effect of devil decline upon the population and individual-level isotopic niche breadths of both Tasmanian devils and spotted-tailed quolls. The extent of devil decline, using time since disease arrival as a proxy, had no effect on population level isotopic niche breadths. However, niches of both species were significantly smaller in areas with high coverage of human-modified habitat. I did not find evidence of differences in individual specialisation between sites. I conclude that anthropogenic influences on resource availability have a larger impact on carnivore niches in this system than top carnivore decline. Finally, I conclude by discussing the key findings of this thesis and placing them within the broader contexts of Tasmanian devil ecology, stable isotope analysis and ecological niche variation. This work demonstrates the robustness of Tasmanian devil isotopic niches to disease and decline, and shows that ecological context is a key driver of isotopic variation among Tasmanian devils. This research also reveals some of the challenges and opportunities afforded in applying stable isotope analysis to marsupials, particularly in Australia, which is thus far an under-utilised method in this taxon and in this region of the world. Ultimately, the work of this thesis contributes to our understanding of ecological niche dynamics, and highlights the need to consider both community-level and landscape-level perturbations when investigating ecological niches in changing communities.
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